1. Field of the Invention
The invention relates to a fuel cell having stable and good power generation characteristics and having good safety, a fuel cell system including the fuel cell and an electronic device equipped with the fuel cell.
2. Description of the Background Art
In recent years, expectations have been growing for fuel cells, because they can provide efficient power sources for portable electronic devices supporting information-oriented society and provide efficient stand-alone power generation devices. In a fuel cell, a fuel and oxygen from the air are electrochemically oxidized and reduced at an anode and a cathode, respectively, and electric power is generated through this reaction. Electrolyzers have also been investigated, which use the reverse reaction in which pure water is decomposed into hydrogen and oxygen through electrolysis.
Among a variety of fuel cells, a polymer electrolyte fuel cell (hereinafter also referred to as “PEFC”) using a solid polymer ion-exchange membrane as an electrolyte has a thin electrolyte membrane and has a relatively low reaction temperature of 100° C. or less, which is lower than that of phosphoric acid or solid oxide fuel cells. Thus, it does not need large accessories so that it can achieve a small-sized fuel cell system.
Among polymer electrolyte fuel cells, a direct methanol fuel cell (hereinafter also referred to as “DMFC”), in which an aqueous methanol solution is supplied to an anode, while protons and electrons are directly taken out of the aqueous methanol solution to generate electric power, does not need a reformer and thus has the potential to achieve a practical compact power source. In the direct methanol fuel cell using a fuel of an aqueous methanol solution, which is liquid at room temperature under normal pressure, the fuel having a high volume energy density can be handled with a simple vessel and without a high-pressure gas cylinder. Thus, it can provide a highly-safe compact power source with a small fuel vessel. Therefore, attention is focused on it from the view point of its application to compact power sources for electronic devices such as portable electronic devices, particularly from the view point of its application to alternative secondary batteries for portable electronic devices. Fuel cells using a liquid fuel also have the potential of using a liquid fuel with higher volume energy density, higher flash point and excellent safety, such as ethanol and propanol, in the future.
In a fuel cell, a fuel and air are electrochemically oxidized and reduced at an anode and a cathode, respectively, to generate electric power. When a liquid fuel such as an aqueous methanol solution is supplied to the anode, the liquid fuel in contact with the anode is oxidized and separated into protons and gas such as carbon dioxide gas. For example, in the case of an aqueous methanol solution, carbon dioxide gas is generated on the anode side according to the reaction: CH3OH+H2O→CO2↑+6H++6e−.
Protons are transferred to the cathode side through an electrolyte membrane. At the cathode, the protons and oxygen from the air react to produce water. In this process, electrons pass through an external load and move from the fuel cell to the cathode and are taken out as electric power.
Since fuel cells using a liquid fuel have the above-mentioned mechanism, the liquid fuel such as an aqueous methanol solution and exhaust gas generated at the anode, such as carbon dioxide gas, form a gas-liquid mixture state at the anode. Thus, it is necessary to provide a gas discharge hole for discharging the exhaust gas out of the fuel cell. In this case, however, there is a problem in which the liquid fuel such as an aqueous methanol solution can leak from the gas discharge hole.
Under the circumstances, there are disclosed techniques for discharging the exhaust gas from a gas-liquid mixture of a liquid fuel such as an aqueous methanol solution and exhaust gas such as carbon dioxide gas.
Japanese Patent Laying-Open No. 2000-106201 discloses a fuel cell including a fuel vaporizing layer and a fuel permeable layer that is stacked on the fuel vaporizing layer and supplies a supplied liquid fuel to the fuel vaporizing layer. In the fuel vaporizing layer, therefore, the gaseous fuel and carbon dioxide are mixed in a vapor phase so that the problem of the liquid fuel leakage can be solved.
However, the structure disclosed in Japanese Patent Laying-Open No. 2000-106201 has a problem in which if a high concentration fuel is used, the discharge of carbon dioxide to the outside of the fuel cell can be accompanied by the discharge of the high concentration gaseous fuel.
Japanese Patent Laying-Open No. 2005-032600 discloses a gas-liquid separation system including a gas-liquid separator that separate a gas-liquid two-layer flow into exhaust gas and a liquid, a carburetor that heats the gas-liquid two-layer flow to vaporize the liquid, a selecting valve that changes the direction of the gas-liquid two-layer flow, a liquid quantity sensor that measures the quantity of a liquid staying in the gas-liquid separator, and a controller that determines the direction of the gas-liquid two-layer flow from the selecting valve based on the liquid quantity measured by the liquid quantity sensor.
However, the configuration disclosed in Japanese Patent Laying-Open No. 2005-032600 still has a problem in which the system is complicated, and separation between the liquid fuel and exhaust gas generated at the anode is difficult.
Japanese Patent Laying-Open No. 2001-102070 discloses a fuel cell including a gas discharge hole that is formed in a fuel vessel or an introducing tube and provided with a separation membrane composed of a porous material having pores whose surface is water-repellant treated. A liquid fuel such as an aqueous methanol solution is supplied to the introducing tube and then supplied to an anode by the capillary phenomenon of a liquid fuel holding part. The fuel cell is configured such that carbon dioxide gas generated at the anode is discharged to the introducing tube through the liquid fuel holding part and discharged through the separation membrane provided on the gas discharge hole to the outside of the fuel cell.
However, the structure of Japanese Patent Laying-Open No. 2001-102070 causes a problem, for example, in a case where an aqueous methanol solution is charged into the introducing tube. Specifically, all the separation membranes provided on the introducing tube are immersed in the aqueous methanol solution and thus increase the internal pressure of the introducing tube without discharging carbon dioxide gas generated at the anode. Therefore, there is a problem in which the aqueous methanol solution can leak out of the fuel cell due to the increase in the internal pressure of the introducing tube, even if the surface of the pores in the porous separation membrane is water-repellent treated with a fluororesin.
There is also a problem in which when an aqueous methanol solution with a high concentration is supplied to the introducing tube, the aqueous methanol solution can infiltrate into the separation membrane because of a surface tension of methanol lower than that of water, so that the separation membrane cannot perform its own function, even if the surface of the pores in the porous separation membrane is water-repellent treated.
Japanese Patent Laying-Open No. 2003-331899 discloses a liquid fuel cell including a liquid fuel storing part provided with a gas-liquid separation hole having a gas-liquid separation membrane, in which the gas-liquid separation membrane is an oil-repellent treated porous fluororesin membrane or an oil-repellent treated laminate composite of a porous fluororesin membrane and another exhaust gas-permeable material. Japanese Patent Laying-Open No. 2003-331899 suggests that if the gas-liquid separation membrane is oil-repellent treated, leakage of a lipophilic liquid fuel such as methanol and ethanol can be prevented.
However, the method of Japanese Patent Laying-Open No. 2003-331899 has a problem in which even if the gas-liquid separation membrane is oil-repellent treated, an aqueous methanol solution with a high concentration in contact with the gas-liquid separation membrane can infiltrate into the membrane so that the function of the gas-liquid separation membrane can be easily degraded.
There is also a problem in which if operation for power generation is performed in an orientation where carbon dioxide less comes into contact with the gas-liquid separation membrane, for example, if the positive electrode is placed upward in the vertical direction, carbon dioxide generated at the negative electrode can increase the internal pressure of the liquid storing part until it comes into contact with the gas-liquid separation membrane so that the aqueous methanol solution can leak from the gas-liquid separation membrane to the outside of the fuel cell.
Japanese Patent Laying-Open No. 2002-175817 discloses a fuel cell including a plate provided with a channel for supplying a liquid fuel such as an aqueous methanol solution and a channel for discharging generated carbon dioxide gas and including a fuel permeable member that allows only a liquid to pass through to the liquid fuel supply channel and a diffusion layer. The fuel permeable member is placed so as to envelop the liquid fuel supply channel, and thus the fuel cell is configured such that generated carbon dioxide gas is not discharged to the liquid fuel supply channel but discharged from a carbon dioxide gas discharge channel to the outside of the fuel cell.
However, the structure disclosed in Japanese Patent Laying-Open No. 2002-175817 has a problem in which as disclosed in its specification, if a deviation is caused between the positions of the stacked liquid fuel supply channel and fuel permeable member by an insufficient location accuracy in fuel cell assembling or by shaking during transportation, carbon dioxide gas can be discharged to the liquid fuel supply channel, or the liquid fuel can leak to the gas discharge channel, so that separation between the aqueous methanol solution and carbon dioxide gas can be difficult.
In addition, if a pressure difference occurs in such a manner that the pressure for the liquid fuel supply to the channel exceeds the pressure for the discharge of carbon dioxide gas, the fuel permeable member can be dissociated from an anode catalyst layer and bent toward the liquid fuel supply channel side. Thus, there is a problem in which if the fuel permeable member is dissociated from the anode catalyst layer, carbon dioxide gas can stay at the interface between the fuel permeable member and the anode catalyst layer to inhibit the liquid fuel supply to the anode catalyst layer.
Japanese Patent Laying-Open No. 2005-235519 discloses a fuel cell structure including a polymer solid electrolyte membrane, anode and cathode electrodes integrally formed on both sides of the polymer solid electrolyte membrane, a fuel diffusion layer placed on the anode electrode side, an air diffusion layer placed on the cathode electrode side, an electrical power collector that is placed on each of the outer sides of the fuel diffusion layer and the air diffusion layer and collects electric energy generated between the anode and cathode electrodes, and a gas-liquid separation membrane placed on the anode electrode side. Japanese Patent Laying-Open No. 2005-235519 suggests that the gas-liquid separation membrane can well discharge carbon dioxide generated in a reaction chamber on the anode side so that the efficiency of the reaction between methanol and water can be improved.
Japanese National Patent Publication No. 2006-507625 discloses a bi-polar plate having two individual fuel cells, which includes: an anode portion in a first fuel cell, wherein the anode portion includes a fuel flow field, an exhaust gas permeable membrane positioned away from the anode aspect of a membrane electrolyte and a gaseous effluent vent channel that is positioned adjacent to the exhaust gas permeable membrane and communicates gaseous effluent from the anode aspect of the membrane electrolyte via an outlet; and a cathode portion in a second fuel cell having a flow field by which oxygen is introduced to the cathode portion of the fuel cell.
In the methods disclosed in Japanese Patent Laying-Open No. 2005-235519 and Japanese National Patent Publication No. 2006-507625, the fuel diffusion layer and the fuel flow field have a gas-liquid mixture state of carbon dioxide and the fuel so that carbon dioxide can be insufficiently discharged in the layer thickness direction by the gas-liquid separation membrane and the exhaust gas permeable membrane and thus can be mixed with the fuel to form a gas-liquid mixture flow and discharged out of the fuel cell. Thus, the fuel cell still has a problem in which the fuel can finally leak outside the fuel cell, when carbon dioxide is discharged with a liquid waste tank or the like. In addition, the fuel supply direction is contrary to the carbon dioxide discharge direction at the anode electrode so that the efficiency of the liquid fuel supply can be easily reduced.
Japanese Patent Laying-Open No. 2004-206885 discloses a fuel cell that includes a fuel chamber that is provided adjacent to an anode in order to suppress an excessive supply of a liquid fuel to the anode, wherein the fuel chamber contains the liquid fuel and a polymer material.
The technique of Japanese Patent Laying-Open No. 2004-206885 has a problem in which the exhaust hole for carbon dioxide generated by the reaction also serves as an injection hole for the liquid fuel, so that separation between exhaust gas generated at the anode and the liquid fuel is difficult, and also has another problem in which the liquid fuel supply efficiency can be easily reduced by the contamination of the liquid fuel with carbon dioxide, and when the liquid fuel is distributed to the whole, the structure for discharging carbon dioxide in the layer thickness direction of the fuel cell makes the liquid fuel travel a long distance, so that the pressure loss can be increased when the fuel is supplied.